The present invention relates to an exposure time controlling system for use in a photographic device.
Generally, in photography, there is a close relation among the factors of: brightness or luminance of an object; sensitivily of a film; shutter speed; and size of an aperture. The relation among these factors can be expressed by a known system which is so-called additive system of photographic exposure, being abbreviated as APEX system. According to the APEX system, the relation can be expressed as follows: EQU Av+Tv=Sv+Bv (1)
in which Av is APEX value of the aperture, Tv is APEX value of the shutter speed, Sv is APEX value of the film sensitivity, Bv is APEX value of the luminance or brightness of the target object. Each of the APEX values is logarithmic values of the well known values. More particularly, Av=2.multidot.log.sub.2 F in which F is a f/stop number, Tv=log.sub.2 (1/T) in which T is the shutter speed, and Sv=log.sub.2 (NS) in which N is a constant value and S is the film sensitivity such as ASA number. When the APEX shutter speed value Tv is calculated by the use of the equation (1), the shutter speed T is calculated by the use of equation T=2.sup.-Tv. Thereafter, the shutter of the photographic device is so controlled as to be opened for a time period of 2.sup.-Tv seconds.
Conventionally, there have been proposed various types of systems for controlling the shutter to be opened for the time period of 2.sup.-Tv seconds, more particularly, for counting the time period of 2.sup.-Tv seconds. By means of a conventional frequency divider, the time period 2.sup.-Tv such as 1/60, 1/125, 1/250, 1/500, etc. can be easily obtained in case that Tv is integer. However, the exposure time available in this case are only by steps of 1 Ev. If a more delicate exposure time control by steps of a fraction of 1 Ev is desired, some complicated construction for obtaining various exposure times may be required.